Radio frequency identification (RFID) systems typically include at least one reader and a plurality of transponders, which are commonly termed credentials, cards, tags, or the like. The transponder may be an active or passive radio frequency communication device which is directly attached to or embedded in an article to be identified or otherwise characterized by the reader. Alternatively, the transponder may be embedded in a portable substrate, such as a card or tag, carried by a person to be identified or otherwise characterized by the reader. An active transponder is powered up by its own internal power supply, such as a battery, which provides the operating power for the transponder circuitry. In contrast, a passive transponder is characterized as being dependent on the reader for its power. The reader “excites” or powers up the passive transponder by transmitting excitation signals of a given frequency into the space surrounding the reader, which are received by the transponder and provide the operating power for the circuitry of the recipient transponder.
Communication between the reader and transponder is enabled by cooperative resonant circuits which are provided in each reader and transponder. The resonant circuit of a reader typically includes an inductor and a capacitor. The capacitor is coupled in series between the inductor and a signal driver. The inductor is usually in the form of an antenna coil which is capable of magnetically coupling to an inductor in the resonant circuit of a compatible transponder through mutual inductance. Communication is initiated when a transponder is proximally positioned relative to the reader. The reader has a power supply which conveys a current to the reader resonant circuit causing the reader antenna to produce an excitation signal in the form of an electromagnetic field. The excitation signal couples to the antenna of the proximally-positioned transponder through mutual inductance and the excitation signal powers and clocks the transponder circuitry initiating operation of the transponder.
Transponder operation comprises generation of a response signal at a specified frequency and transmission of the resulting transponder response signal back to the reader. In particular, the transponder resonant circuit receives a current in response to the excitation signal which causes the transponder antenna to produce a response signal in the form of an electromagnetic field. The response signal couples to the reader antenna through mutual inductance in substantially the same manner as described above with respect to coupling of the excitation signal to the transponder antenna.
The transponder modulates the response signal to encode data stored in the memory of the transponder circuitry into the response signal. When the response signal couples to the reader antenna, a corresponding voltage is induced in the reader antenna at the specified frequency. The reader processes the induced voltage to read the data encoded in the response signal. The resulting data may be communicated to an output device, such as a display, printer, or storage device, and simultaneously, or alternatively, communicated to a host computer, if a host computer is networked into the RFID system.
An important operating parameter of the reader is the range of the reader when communicating with a transponder. The range of the reader is inter alia strongly affected by the strength of the electromagnetic field generated by the reader resonant circuit. In order to generate a field strength which provides the reader with adequate range, the designer of the reader must properly specify a resonant circuit which is appropriately tuned to a predetermined frequency for the desired application of the RFID system.
The range of the reader is often altered by the characteristics of the operating environment in which the reader resides. In a typical case where the reader is mounted in a fixed location on a support structure, the range of the reader is susceptible to the materials of the mounting location and other objects within the operating environment. For example, if the mounting location of the reader is in an operating environment which includes nearby metal, the metal can detune the resonant circuit of the reader from the predetermined frequency and dramatically reduce the range of the reader. The presence of one or more transponders in the operating environment of the reader which have variations in their respective resonant circuits due to imprecise manufacturing tolerances can detune the resonant circuit of the reader. Imprecise manufacturing tolerances in the resonant circuit of the reader can also detune the resonant circuit of the reader.
One means of overcoming the above-mentioned problems is to provide the reader resonant circuit with an adjustable tuning capability so that the tuning of the reader resonant circuit is adjusted in response to variations in its operating environment. As such, the present invention recognizes a need for a reader of an RFID system which is adaptable to variations in its operating environment. Accordingly, it is generally an object of the present invention to provide an RFID system having a reader which exhibits satisfactory performance characteristics while adjusting to variations in a given system operating environment. More particularly, it is an object of the present invention to provide a reader achieving a uniformly satisfactory level of performance when the reader is employed in different operating environments. It is another object of the present invention to provide a reader having a resonant circuit which automatically retunes itself to maintain a desired performance level in response to variations in the operating environment of the reader. These objects and others are accomplished in accordance with the invention described hereafter.